GLOSSARY – M

GLOSSARY – M

MAFIC (magnesium and ferric)
An adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite, high in magnesium and iron but low in silica.
[see – FELSIC]

MAGNESIOFERRITE, MgFe2O4
A rare spinel mineral crystallizes as black metallic octahedral crystals and has been documented in the recrystallized impact breccias of the Steen River Impact Structure (SRIS). It is named after its chemical composition of magnesium and ferric iron. The density is 4.6 – 4.7 (average = 4.65), and the diaphaniety is opaque.

MAGNETIC ANOMALIES
In geophysics, a magnetic anomaly is a local variation in the Earth’s magnetic field resulting from variations in the chemistry or magnetism of the rocks. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study. Mapping of variation over an area is valuable in detecting structures obscured by overlying material.

MAJORITE,Mg3(MgSi)(SiO4)3
A type of garnet mineral found in the upper mantle of the Earth. It is distinguished from other garnets in having Si in octahedral as well as tetrahedral coordination. Majorite was first described in 1970 from the Coorara Meteorite of Western Australia and has been reported from various other meteorites in which majorite is thought to result from an extraterrestrial high pressure shock event. Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.

MASKELYNITE
A clear, glassy pseudomorph of plagioclase produced by a relatively low pressure (250-300 kilobars) and low temperature (350°C) shock wave. It is found in the rocks of the central peaks of Clearwater West and Manicouagan craters, Quebec, Canada. Heated maskelynite reverts to crystalline plagioclase, indicating only slight structural disordering unlike fused plagioclase glass.NATURAL TERRESTRIAL MASKELYNITE, Dence, et al THE AMERICAN MINERALOGIST 1967

METAMORPHIC ROCK
Rock that was formed by the recrystallization of a pre-existing rock in response to a change of mainly temperature and pressure (metamorphism). Metamorphic rocks are, e.g., marble (metamorphic limestone), gneiss, schist.
[see – SHOCK METAMORPHISM]

METEOR
Incoming meteoroids enter the earth’s atmosphere at 11 km/sec to 72 km/sec. Ram pressure between the air and the object create a very high temperature plasma at the front of the meteor. This plasma becomes visible at between about 120 km and 75 km above the earth. Energy goes into melting and vaporizing stone and metal. Energy is shed as material ablates. In a couple of seconds most meteors are have been consumed. The left-over debris is called meteoric dust or just meteor dust.

METEORITE
If a meteoroid’s size, composition, speed and entry angle allow it to survive the “meteor” phase of entry, it will slow to about 4 km/sec and enter “dark flight” at 20 km to 15 km above earth. Light emission from incandescence and ion recombination ceases. The meteor will arch into a more vertical trajectory, slow to terminal velocity of about 0.1 km/sec and fall as a meteorite.
If the meteoroid is of sufficient size to keep it’s hyper-velocity >12 km/sec through the atmosphere becoming an impactor, it will impact the ground and explode. The kinetic energy of an object of mass m traveling at a speed v is = (½)mv2, provided v is much less than the speed of light.
[see – METEORITE]

METEORITE: Organic
Direct evidence of complex prebiotic chemistry from a water-rich world in the outer solar system is provided by the 4.5-billion-year-old halite crystals hosted in the Zag and Monahans (1998) meteorites. This study offers the first comprehensive organic analysis of the soluble and insoluble organic compounds found in the millimeter-sized halite crystals containing brine inclusions and sheds light on the nature and activity of aqueous fluids on a primitive parent body.Organic matter in extraterrestrial water-bearing salt crystals
Chan et al
[see – METEORITE]

METEORITE CRATER
[see – CRATER (ASTROBLEME)]

METEOROID (smaller than ASTEROIDS)
A meteoroid is a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom.
[see – ASTEROID, METEOR, METEORITE.]

MICAS
Group of sheet-silicate minerals; e.g., muscovite, biotite. Kink bands in mica may be produced by shock.

MODIFICATION STAGE
The initial transient crater is unstable and the modification stage commences. Small craters of <4 km (on Earth) are relatively stable after the excavation stage. For larger craters, the impact structure is gravitationally unstable and its modification stage will include uplift of the crater floor and collapse of the unstable steep walls (slumping). These movements will be completed in a few minutes and could result in a complex or multi-ring crater. Minor faulting, mass movement and/or hydrothermal activity in the larger craters could last indefinitely.
[see – CRATER CLASSIFICATION]
[see – CRATER FORMATION – Contact & Compression]
[see – CRATER IDENTIFICATION]

MOSAICISM
Shock feature in mineral crystals. Under crossed polarizers of the polarization microscope, mosaicism shows as a checkered, mosaic-like extinction pattern resulting from shock-induced disorder in the crystal lattice.

MULTIPLE IMPACT
Synchronous impact of two (paired impact) or more impactors. A Late Triassic multiple impact has been proposed to have produced a chain of five large impact structures on the European and the American continents.

Compilation of selected terrestrial meteorite impacts during the Triassic and the postulated Late Triassic multiple impact theory, modified after Spray et al.(1998). Lucas et al.(2012)suggested an age of ∼220 Ma for the Carnian/Norian boundary, which has an age of ∼227Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data from Koeberl et al.(1996), Ramezani et al.(2005), Schmieder and Buchner (2008), Schmieder et al.(2010).